A polybenzazole fiber has tensile strength and tensile modulus twice or more greater than those of polyparaphenylene terephthalamide fiber which is a representative super fiber currently available in the market. Thus, this fiber is expected to be a next generation super fiber.
This fiber has been known to be obtained from a polyphosphoric acid solution of polybenzazole polymer. For example, spinning conditions are technically disclosed in U.S. Pat. No. 5,296,185 and U.S. Pat. No. 5,385,702, washing with water and drying is technically disclosed in WO94/04726, and heat treatment is technically disclosed in U.S. Pat. No. 5,296,185.
The tensile modulus of a high tensile strength polybenzazole fiber prepared by the above-mentioned conventional production method only reaches 290 GPa at most, even by a heat treatment at a temperature of not less than 350.degree. C. as disclosed in U.S. Pat. No. 5,296,185. Despite the extremely high tensile modulus reported to have been achieved in a laboratory, yarn (collective filaments) having a tensile strength of not less than 5.0 GPa and a tensile modulus of not less than 290 GPa cannot be readily produced at an industrial level, except for an isolated case where molecular relaxation was suppressed under specific spinning conditions (Japanese Patent Unexamined Publication No. 325840/1996).
Thus, the present invention aims at providing a technique enabling easy production of a polybenzazole fiber having an ultimate tensile modulus as an organic fiber material.
An ultimate property of a fiber has been captured using a rigid polymer such as a so-called ladder polymer. Such rigid polymer, however, has no flexibility. For flexibility and processability of an organic fiber to be achieved, a linear polymer should be essentially used.
As suggested by S. G. Wierschke et al. in Material Research Society Symposium Proceedings, vol. 134, p.313 (1989), a linear polymer having the highest theoretical tensile modulus is a cis-type polyparaphenylene benzobisoxazole. This has been confirmed by Tashiro et al. in Macromolecules, vol. 24, p.3706 (1991). It has been considered that, of a series of polybenzazole polymers, a cis-type polyparaphenylene benzobisoxazole has an ultimate primary structure, since it has a crystal modulus of 475 GPa (P. Galen et al., Material Research Society Symposium Proceedings, vol. 134, p.329 (1989)). Therefore, a theoretical conclusion will be to use polyparaphenylene benzobisoxazole as a polymer to achieve an ultimate tensile modulus.
This polymer can be made into a fiber by the method described in U.S. Pat. No. 5,296,185 and U.S. Pat. No. 5,385,702, and a heat treatment is applied by the method described in U.S. Pat. No. 5,296,185. The yarn thus obtained has a tensile modulus of 290 GPa at most, which corresponds to only 61% of the theoretical crystal modulus. Further studies done by the present inventors in an attempt to provide a fiber having properties closer to the theoretical values have resulted in a polybenzazole fiber having a higher tensile strength and a higher tensile modulus, as well as a process for easy manufacture of the fiber at an industrial scale.